Production of tunable coherent radiation through parametric amplification from a fixed frequency laser beam is effected through a device known as an optical parametric oscillator (OPO). The theoretical rationale and complexities associated with parametric amplification and OPOs are well known to those skilled in the art.
In a conventional OPO, the OPO receives a beam of laser radiation at a pump frequency .omega..sub.p from a pump source. The pump frequency .omega..sub.p is received into a resonant optical cavity, wherein pump frequency .omega..sub.p is directed through a non-linear medium, usually a crystal, located within the resonant cavity. As a result, two lower energy signals are generated from the pump beam, which are known as the signal, at frequency us and the idler at frequency .omega..sub.i.
The content and orientation of the crystal and the design of the resonant cavity determines the signal .omega..sub.s and idler .omega..sub.i frequencies. The feedback within the resonant cavity causes gain in the parametric waves, a process similar to build-up in a laser cavity. The cavity can either be singly-resonant in which end mirrors reflect only signal frequency .omega..sub.s, or doubly-resonant in which end mirrors reflect both signal .omega..sub.s and idler .omega..sub.i frequencies. End mirrors of the OPO are transparent to the pump frequency .omega..sub.p. OPOs with singly-resonant cavities are typically more stable in their output than OPOs with doubly-resonant cavities.
Due to the nature of the non-linear crystal and the conversion process, the pump frequency .omega..sub.p is always higher than the frequency of the signal .omega..sub.s and the idler .omega..sub.i. The sum of the signal .omega..sub.s and idler .omega..sub.i frequencies is equal to the pump frequency .omega..sub.p.
Power and energy conversion efficiency to the idler frequency .omega..sub.i in an OPO is limited by the quantum efficiency and photon efficiency. Since idler frequency .omega..sub.i is less than half of the pump frequency .omega..sub.p, the quantum limit is always less than one half and significantly less so when the idler frequency wi is far from degeneracy. Furthermore, for pulsed OPOs, pump regeneration from signal .omega..sub.s and idler .omega..sub.p frequency reduces photon conversion efficiency due to temporally and/or spatially varying pump radiation. Nevertheless, idler output provides a useful means of generating coherent radiation in spectral regions that are difficult to access by other sources.
There are a variety of types of crystals that may be used in OPOs for various spectral regions. In particular, non-linear optical crystals capable of producing parametric output which have been developed for commercial applications, include, but are not limited to, potassium titanyl phosphate (KTP), potassium titanyl arsenate (KTA), lithium niobate (LiNbO.sub.3), potassium niobate (KNbO.sub.3), silver gallium selenide (AgGaSe.sub.2), and silver gallium sulfide (AgGaS.sub.2). When a fixed laser is used to generate tunable waves from certain crystals, an electric field may be applied to the crystal, or the crystal may be temperature or angle tuned, or a combination of electrical voltage, temperature and/or angle tuning is required to achieve phase matching.
Periodically poled LiNbO.sub.3 (PPLN) has been shown to be particularly well-suited for OPO wavelength generation in the 1.4-4.0 .mu.m region due to its low threshold, large non-linear coefficient, large acceptance angle, absence of walk-off, and transparency in this region (L. E. Myers, R. C. Eckardt, M. M. Fejer, R. L. Byer, W. R. Bosenberg, and J. W. Pierce, J. Opt. Soc. Am. B12, 2102-2116 (1995)). Although continuous wave OPOs utilizing PPLN have demonstrated high conversion efficiencies (W. R. Bosenberg, A. Drobshoff, J. I. Alexander, L. E. Myers, and R. L. Byer, Opt.Lett. 21, 1336-1338 (1996)), typically pulsed OPOs have not yet approached continuous wave OPO efficiencies due to factors such as back conversion of the pump wave and non-uniform pump depletion.
In a typical configuration of an OPO using a crystal or PPLN medium, the crystal or PPLN is located between the two cavity mirrors. Light is directed through the entry mirror through the crystal or PPLN medium and through the exit mirror with certain frequencies being reflected back into the cavity to again be transmitted through the crystal or PPLN medium.
Other techniques of increasing conversion efficiency in similar OPO configurations suggest the inclusion of a second crystal or PPLN medium located within the cavity, and situated between the two cavity mirrors. In these structures, an entry mirror receives the light which directs the beam through a first crystal or PPLN to be received by a second crystal or PPLN and then on to an exit mirror. Again, the exit mirror transmits certain frequencies while reflecting other frequencies back through the crystal media.
Conversion schemes using tandem and intercavity difference frequency mixing (DFM) OPOs have been proposed and analyzed (K. Koch, G. T. Moore, and E. C. Cheung, J. Opt, Soc. Am. B 12, 2268-2273 (1995); and G. T. Moore and K. Koch, IEEE J. Quantum Electron. 32, 2085-2094 (1996)) and may help mitigate some of the limitations inherent in pulsed OPOs, however, such suggested approaches fail to significantly increase conversion efficiency.
Frequency conversion schemes utilizing multiple crystals within the OPO cavity demonstrating the OPO-DFM system applying two separate PPLN crystals are discussed and analyzed in J. M. Fukumoto, H. Komine, W. H. Long et al. Advanced Solid State Lasers (1998) (Optical Society of America, Washington, D.C., 1998), post deadline paper PDP-4, where a factor of two increase in the idler conversion efficiency is demonstrated.
Reference may be had to the following patents for further information concerning the state of the technology relating to OPOs (all of the references are incorporated herein by reference):
U.S. Pat. No. 5,400,173, issued Mar. 21, 1995 entitled "Tunable Mid-Infrared Wavelength Converter Using Cascaded Parametric Oscillators" to Komine, describes an apparatus for converting a fixed wavelength pump into a plurality of spectral output beams. The first resonator is coupled to a first non-linear optical crystal for turning said pump into a first and second output beams.
U.S. Pat. No. 5,500,865, issued Mar. 19, 1996 entitled "Phased Cascading Of Multiple Non-linear Optical Elements For Frequency Conversion", to Chakmakjian, uses two or more crystals in tandem to increase the interaction length of the non-linear optical process for improved efficiency. Additional optical components are inserted into the optical path to adjust the phase delay of the interacting waves in order to maintain coherent generation of the product radiation.
U.S. Pat. No. 4,639,923, issued Jan. 27, 1987, entitled, "Optical Parametric Oscillator Using Urea Crystal", to Tang, et al., uses a crystal of urea as the non-linear optical medium for constructing an OPO.
U.S. Pat. No. 5,159,487, issued Oct. 27, 1992, entitled "Optical Parametric Oscillator OPO Having A Variable Line Narrowed Output", to Geiger et al., describes an OPO that includes a pump laser for producing a pump beam; an optical resonator; an OPO crystal disposed within the optical resonator aligned with and responsive to the pump beam to produce a parametrically generated output; and a device external to the optical resonator for line narrowing the parametrically generated output.
U.S. Pat. No. 5,144,630, issued Sep. 1, 1992, entitled "Multiwavelength Solid State Laser Using Frequency Conversion Technique", to Lin, describes an apparatus for producing multiwavelength coherent radiations ranging from deep ultraviolet to mid-infrared. The basic laser is a pulsed Nd:YAG or Nd:YLF laser which is frequency converted by a set of novel non-linear crystals including D-CDA, LBO, BBO, KTP and KNbO.sub.3 where efficient schemes using noncritical phase matching and cylindrical focussing are employed.
U.S. Pat. No. 5,117,126, issued May 26, 1992, entitled "Stacked Optical Parametric Oscillator", to Geiger, describes a stacked OPO wherein two optical parametric crystals are coaxially disposed in a single resonator, Incident radiation is coupled to the resonator and causes parametric oscillations of the two crystals. The two crystals are independently tuned, such as by angular orientation to produce distinct components of secondary radiation.
U.S. Pat. No. 5,079,445, issued Jan. 7, 1992, entitled "High Output Coupling Cavity Design For Optical Parametric Oscillators", to Guyer, discloses a cavity design for use with a non-linear medium which may be used as an oscillator using pump energy with frequency (FP) interacting with the non-linear medium for parametrically generating outputs having a signal frequency (FS) and an idler frequency (FI). The parametric radiation which is produced satisfies the relationship which is common for optical parametric amplifiers and oscillators, FP=FS+FI.
U.S. Pat. No. 5,070,260, issued Dec. 3, 1991, entitled "Ultrahigh-Resolution Optical Parametric Oscillator Frequency Measurement and Synthesis System", to Wong, discloses one or more OPOs which are arranged selectively, singly, serially, and/or in parallel and each OPO is responsive to an input pump beam having a fractional stability to produce output signals and idler beams having fractional stabilities that correspond to or are better than the fractional stability of the pump beam and in such a way that the sum of the frequencies of the output signal and idler beams of each OPO is constrained to be equal to the frequency of the input beam thereof.
U.S. Pat. No. 5,047,668, issued Sep. 10, 1991, entitled "Optical Walkoff Compensation In Critically PhaseMatched Three-Wave Frequency Conversion Systems". to Bosenberg, discloses a walkoff-compensation frequency conversion system such as an OPO including a pair of non-linear crystals such as: Beta-Barium Metaborate, aligned in an optical cavity with their optical axis at an angle with respect to the axis of the cavity.
U.S. Pat. No. 4,884,277, issued Nov. 28, 1989, to Anthon, et al., discloses an intracavity frequency-modified laser of improved amplitude stability which is obtained through the use of a plurality of non-linear optical crystals within the laser cavity.
The efficiency for down-conversion in an OPO is limited by reversion of the generated waves back into the pump. Unlike up-conversion, such as second harmonic generation, where the efficiency increases monotonically as the pump intensity is increased and is limited only by material damage, in a parametric oscillator the conversion process is reversible. As the power in the signal and idler waves approaches that of the incident pump wave, the pump field amplitude can be driven through zero and begin to grow again in magnitude at the expense of the longer wavelengths.
As a result of this back-conversion, the efficiency of an OPO goes through a maximum as the pump intensity, or gain, is increased. If the pump is a continuous plane wave, the maximum photon efficiency is 100 percent at (.pi./2).sup.2 =2.5 times threshold. However, for a pulsed Gaussian beam, the efficiency depends on the ratio of the pulse length to the cavity length. For a pulse length (FWHM) equal to 40 cavity round-trip (RT) times, the maximum efficiency is 29% at about 2 times threshold.
For conversion to a single wavelength, the photon efficiency must be multiplied by the quantum efficiency, or the ratio of the pump wavelength to the desired wavelength. This is because in a conventional OPO, each pump photon produces one signal photon and one idler photon. If the wavelength of the selected photon is much longer than the pump, then the quantum efficiency is low. If the signal and idler are equal, then the OPO is degenerate and the net efficiency is doubled, but now the device is no longer tunable for a fixed pump wavelength.
One way to reduce back-conversion and improve the efficiency of the OPO is to reflect the pump wave back into the medium after its first pass (Care must be taken not to allow any idler feedback). In this way, not only is the small signal gain doubled (reducing build-up time), but back-conversion to the pump is reduced as the second pass must generate its own idler from zero. For the case quoted above (FWHM=40RT), the efficiency increases from a maximum of 29% to 52% at about 2.5 times threshold.
Conversion efficiency to the signal wave can be further improved by dividing the crystal into multiple segments and rejecting the idler at each break. Since the signal and pump must repeatedly generate new idler, the back-conversion process is minimized. With this approach, depletion of the pump is nearly complete and conversion to the signal approaches the quantum limit at high gains.
Other conversion schemes propose a signal (resonated) wave which is limited by a non-linear output coupler. When the signal exceeds some level, the effective output coupling increases to clamp the field of the resonating wave at a nearly constant value. The non-linear output coupler is basically another parametric medium within the OPO cavity which is tuned to convert the signal wave to the idler wavelength and a new wavelength which is the difference of the signal and idler.
It is evident that it would be desirable to overcome the disadvantages of the stated art by providing an apparatus that allows for the efficient down conversion of coherent light to longer wavelengths.